Silicon clathrates, which exhibit a direct or near-direct bandgap, represent a promising alternative to conventional diamond cubic silicon for optoelectronic applications. They combine the elemental abundance and nontoxicity of Si with a more favorable bandgap and higher optical absorption at room temperature. Their open cage-like frameworks allow for reversible guest atom incorporation with minimal lattice distortion, making them attractive not only for optoelectronics but also for energy storage applications. However, conventional synthesis methods typically require the use of sodium as a structure-directing agent to stabilize the metastable clathrate lattice during thermal decomposition. The resulting sodium incorporation degrades the optoelectronic properties and complicates device integration. Herein, a nontoxic, scalable, vacuum-annealing approach is utilized that effectively removes sodium from silicon clathrates films without the use of hazardous chemicals, such as iodine, HF, or SF6. Optoelectronic characterization reveals a significant improvement in semiconducting behavior, including enhanced surface photovoltage response, marking a critical step toward developing sodium-free silicon clathrate thin films as efficient light absorbers. This vacuum annealing process is compatible with large-area substrates and offers a key step toward a safer, and more sustainable approach to integrating exotic Si phases into the next-generation optoelectronic devices.

We investigate the ultrafast dynamics of excitons in a 2.6 nm-thick ZnO/Zn0.84Mg0.16O quantum well grown on a c-axis sapphire substrate, using non-degenerate time-resolved pump–probe spectroscopy. A pump pulse at 266 nm generates photocarriers within the ZnMgO barriers, and their dynamics is monitored through time-resolved differential reflectance measurements using a supercontinuum probe spanning the 345–400 nm spectral range. Photocarriers generated in the barriers rapidly relax into the quantum well, where they form excitons within sub-picosecond timescales. These excitons quickly thermalize and become localized, likely due to interface disorder or well-width fluctuations, as supported by photoluminescence measurements showing a clear Stokes shift and the absence of free exciton emission. A phonon-assisted absorption process, leading to the effective thermalization of excitons, is observed and analyzed. We identify moreover a negative differential reflectance feature as a photoinduced absorption into a biexciton state, with a binding energy ranging from 18 to 22 meV depending on temperature.

Differential optical transmission experiments performed on TiO2 layers reveal a carrier population decay that follows a power-law over several orders of magnitude in both time and signal amplitude, across a temperature range from 10 K up to room temperature. The exponent of the power-law is fractional, increasing linearly at low temperatures before following an activation law above 160 K. We propose a mechanism based on collision-assisted recombination, where self-trapped excitons are first delocalized and then partially ionized into free polarons as the temperature increases, giving rise to the acceleration of the population decay.

The photoluminescence emission spectrum at low temperature of the TiO2 anatase shows a broad band around 2.3 eV that has been largely attributed to self-trapped excitons. In this paper, time-resolved experiments conducted at low temperature on a thin layer and a single crystal give insights into the carrier dynamics inside TiO2. The decay of the photoluminescence with time can be described by a single power law from the picoseconds time range up to the milliseconds time range. The decay rate increases with increasing temperature. We propose to interpret these observations by a collision mechanism between self-trapped excitons that diffuse via hopping processes. These excitons, which are stabilized by the indirect band gap of anatase TiO2, can be ionized at higher temperatures and lead thus to an acceleration of the observed dynamics.

Three-dimensional (3D) hybrid perovskites (HPs) are currently a key player in optoelectronic applications and, beyond the traditional photovoltaic applications, have also other great potentials as light-emitting diodes, lasers, photocatalysts, and photo- and X-ray detectors. However, despite the excellent performance of 3D HPs, certain HPs such as methylammonium lead iodide (CH3NH3PbI3 named MAPbI(3)) lack long-term environmental stability because they are very sensitive to humidity and degrade under oxygen and light exposures. Moreover, MAPbI(3) films or single crystals or powders are mainly produced from precursor mixture methods and suffer some drawbacks such as the formation of intermediate phases with solvents affecting their performance. To face some of those problems, we have optimized the synthesis of MAPbI(3) via a green solvent-free mechanosynthesis process, which also provides large amounts of powders, and to enhance its chemical stability, we have mixed MAPbI(3) with graphite, which also granted the possibility to combine the different electronic properties of both components. The mechanosynthesis parameters have been optimized for both systems to minimize the defect amount, and their structural, optical, and electronic properties have been characterized. High-resolution transmission electron microscopy (HRTEM) and in situ HRTEM highlighted common crystallographic orientations between MAPbI(3) nanograins (similar to 10 nm) in aggregates of mechanosynthesized (MS) powders. The band gap values and photoluminescence (PL) properties of mechanosynthesized MAPbI(3) are in good agreement with the ones reported for MAPbI(3) synthesized by chemical methods. A decrease in PL intensity related to the introduction of defects is observed with a longer grinding time, confirming that the selected grinding conditions were optimal. These mechanosynthesis conditions of MAPbI(3) were found also suitable to obtain MAPbI(3)@Graphite composites with different amounts of graphite. A homogeneous distribution of few-layer graphite within the MAPbI(3) matrix was noticed in powder-compacted wafers. Graphite-enhanced Raman scattering was observed with the composite with 5 wt % graphite showing its strong resistance to photodegradation and suggesting a charge transfer between graphite and MAPbI(3). Such charge transfer is also in agreement with the quenching of the PL with an increasing amount of graphite. Further analysis of time-resolved PL decays confirmed, supported by X-ray detection results, that the presence of graphite impacted the charge transfer. The presence of graphite would induce a local modification of the Coulomb interaction, "attracting" the charge carriers at the graphite and MAPbI(3) interfaces and thus favoring the charge separation and transport. In addition, the composite with 5 wt % graphite exhibited promising photodetection properties with a photoresponsivity of 1.5 x 10(3) A/W and an effective quantum effect of around 4.4 x 10(6) % at 400 nm with a light power of 330 nW. This high photoconductive gain matches with the trapping of one type of charge carrier, while the other one is multiplied via injection from the contact electrodes, thus contributing even more to the photoconductivity. These results highlighted that the mechanosynthesis, which is green and efficient in providing large amounts of powders, is suitable to design homogeneous MAPbI(3)@Graphite composites with strong interactions between graphite and MAPbI(3) favoring an improved charge transfer and amplification of the photocurrent that will be very promising for photodetection applications.

We study the electron-hole dynamics of c-plane GaN/AlN quantum dots (QDs) emitting above the bulk GaN bandgap [1] by means of time-resolved photoluminescence (TRPL) [2]. The PL dynamics displays a bi-exponential decay with a short time-constant (0.3 ns), independent of photon energy, and a longer one that follows the expected evolution of the radiative lifetime with the QD size. We attribute the fast dynamics to a non-radiative energy transfer to deeper levels, which have sufficiently low density and long lifetime such that their population can be easily saturated. A very long component, showing up as a constant background in our experiment, can thus be explained by the corresponding reverse process: carriers having experienced an ionization and/or trapping event will remain in that state during a long time period before slowly relaxing to their original state. The intrinsic radiative recombination process is recognizable thanks to the characteristic dependence of its lifetime with the emission energy, due to the presence of the large built-in electric field typical of such polar quantum heterostructures [3]. By analytically modeling TRPL spectra, we show that the QD luminescence undergoes a linear redshift with increasing time-delay accompanied by a decrease of its linewidth, while the initial Gaussian lineshape is conserved. From this analysis, we not only demonstrate that the experiment is performed in such excitation conditions such that the dynamical screening of the built-in field by free carriers is negligible, but we also determine the energy-dependence of the QD radiative lifetime with an improved precision when compared to the usual procedure that consists in fitting the PL decays for different QD energies. Beyond the confirmation that the dependence of the effective PL lifetime with emission energy in GaN/AlN QDs is dominated by the built-in electric field [3,4], we also show, using variational calculation, that changes in the QD emission energy by several hundreds of meV within a given QD sample can be understood through fluctuations in the lateral dot dimensions within a given QD subset corresponding to a well-defined dot height. Numerical simulations, that show good qualitative agreement with experimental data over an energy range of 1 eV and six orders of magnitude for the effective PL lifetime when assuming an electric field of 7 MV/cm, enable to evaluate the dot height in our sample to be distributed around a mean value of 1.5 nm [2]. Bibliography: [1] S. Tamariz, G. Callsen, and N. Grandjean, Applied Physics Letters 114, 082101 (2019). [2] M. Hrytsaienko, M. Gallart, M. Ziegler, O. Crégut, S. Tamariz, R. Butté, N. Grandjean B. Honerlage and P. Gilliot, Journal of Applied Physics 129, 054301 (2021) [3] T. Bretagnon, P. Lefebvre, P. Valvin, R. Bardoux, T. Guillet, T. Taliercio, B. Gil, N. Grandjean, F. Semond, B. Damilano, A. Dussaigne, and J. Massies, Phys. Rev. B 73, 113304 (2006). [4] S. Kako, M. Miyamura, K. Tachibana, K. Hoshino, and Y. Arakawa, Applied Physics Letters 83, 984 (2003)

The compound [BiCNIm]3[DyCl6] was synthesized from a nitrile-functionalized imidazolium ionic liquid [BiCNIm][Cl] and DyCl3 ⋅ 6H2O in acetonitrile using solvothermal conditions. Structural characterization reveals that the Dy3+ ions are in a quasi-regular octahedral environment, formed by six chloride anions. The magnetic study indicates that this mononuclear compound exhibits a Single Ion Magnet behaviour. This behaviour is compared to that of other mononuclear compounds containing Dy3+ ions in various octahedral environment.

In order to discuss the exciton fine-structure of transition-metal dichalcogenides mono-layers, excitons are first defined in the subspace of electron- and hole states, including the lowest conduction band (LCB) and the uppermost valence band (UVB). Both bands are spin degenerate at the Gamma-point. All other states are neglected. The resulting exciton states are analyzed in the framework of an invariant expansion of a model Hamiltonian: The spin-orbit coupling in the conduction- and valence band is simulated by introducing a fictive magnetic field, giving rise to a splitting of the electron- and hole states outside the $\Gamma$-point. Then the electron-hole exchange-interaction is introduced into the exciton Hamiltonian. It is due to the fact that electron and hole are indistinguishable particles in the exciton problem. In D3h crystal symmetry this electron-hole exchange-interaction has two different contributions: While a first term accounts for an energy re-normalization of all exciton states, a second term does not influence the optical active (spin-singlet) states but affects only the optical inactive (spin-triplet) states, which become mixed in-between the different exciton series.

The synthesis and characterization of six new lanthanide networks [Ln(L)(ox)(H2O)] with Ln = Eu3+, Gd3+, Tb3+, Dy3+, Ho3+ and Yb3+ is reported. They were synthesized by solvo-ionothermal reaction of lanthanide nitrate Ln(NO3)3·xH2O with the 1,3-bis(carboxymethyl)imidazolium [HL] ligand and oxalic acid (H2ox) in a water/ethanol solution. The crystal structure of these compounds has been solved on single crystals and the magnetic and luminescent properties have been investigated relying on intrinsic properties of the lanthanide ions. The synthetic strategy has been extended to mixed lanthanide networks leading to four isostructural networks of formula [Tb1−xEux(L)(ox)(H2O)] with x = 0.01, 0.03, 0.05 and 0.10. These materials were assessed as luminescent ratiometric thermometers based on the emission intensities of ligand, Tb3+ and Eu3+. The best sensitivities were obtained using the ratio between the emission intensities of Eu3+ (5D0→7F2 transition) and of the ligand as the thermometric parameter. [Tb0.97Eu0.03(L)(ox)(H2O)] was found to be one of the best thermometers among lanthanide-bearing coordination polymers and metal-organic frameworks, operative in the physiological range with a maximum sensitivity of 1.38%·K−1 at 340 K.

A ZnO/PBA/Fe3−δO4 nanocomposite displays enhanced magnetic and optical properties as a result of dual synergy.

We propose an analysis of the emission properties of anatase and rutile titanium dioxide (TiO2) that emphasizes the role of the strong electron-phonon interaction. We performed measurements of photoluminescence (PL) spectra of bulk monocrystals under continuous wave-laser excitation and of their temperature dependence. We show that in both anatase and rutile, weakly bound self-trapped excitons are actually made out from carrier polarons and give rise to a broad emission band in the visible spectral range. The thermal activation of carrier motion allows their hopping to distant sites that leads to the observed quenching of luminescence. In the specific case of rutile TiO2, the PL spectral shape and its intensity-quenching scenario reveal the presence of dark trap states. Moreover, an additional narrow line structure shows up at low temperatures. The latter is due to localized impurity states that can be attributed to oxygen vacancies and can be fitted with a large Huang-Rhys parameter S = 2.5 within a Franck-Condon model. Both phases show thus a very strong interaction between the photogenerated carriers and the lattice.

Copper-and cobalt-based layered simple hydroxides (LSH) are successfully functionalized by a series of fluorene mono- and diphosphonic acids, using anionic exchange reactions and a preintercalation strategy. The lateral functionalization of the fluorene moieties has only little impact on the overall structure of the obtained layered hybrid materials but it influences the organization of the molecules within the interlamellar spacing. For bulky fluorene (9,9-dioctyl derivative), luminescence is preserved when inserted into copper and cobalt hydroxydes, whereas it is completely quenched for the other fluorenes. Detailed characterization of the internal structure and chemical bonding properties for copper-and cobalt-based hybrids is performed via ancillary experimental techniques. For the copper-based LSH class, for which more elusive findings are found, first-principles molecular dynamics simulations unravel the fundamental stabilizing role of the H-bonding network promoted within the local environments of the fluorene mono-and diphosphonic acids. The cobalt series of compounds constitute a new class of hybrid magnets, with ordering temperatures ranging from 11.8 to 17.8 K and show a clear magnetoelectric effect. This effect appears above a threshold magnetic field, which is null below the magnetic ordering temperature, and it persists in the paramagnetic regime till about 110 K.

The syntheses and characterization of four new hybrid coordination networks based on lanthanide ions (Ln = Nd, Sm) and 1,3-carboxymethylimidazolium (L) salt in the presence of oxalic acid (H2ox) are reported. The influence of the synthesis parameters, such as the nature of the lanthanide ion (Nd3+ or Sm3+), the nature of the imidazolium source (chloride [H2L][Cl] or zwitterionic [HL] form) and the presence or not of oxalic acid (H2ox), is discussed. In the presence of oxalic acid, the samarium salt gives only one compound [Sm(L)(ox)(H2O)]·H2O, whatever the nature of the imidazolium ligand, while the neodymium salt leads to three different compounds, [Nd(L)(ox)(H2O)]·H2O, [Nd(L)(ox)0.5(H2O)2][Cl] or [Nd2(L)2(ox)(NO3)(H2O)3][NO3], depending on the imidazolium ligand. In the absence of oxalic acid, gels are obtained, except for the reaction between the neodymium salt and [H2L][Cl], which leads to [Nd(L)(ox)(H2O)]·H2O. All compounds crystallized and their structures were determined by single crystal diffraction. The description of these new phases was consistently supported by ancillary techniques, such as powder X-ray diffraction, thermal analyses and UV-visible-near infrared spectroscopy. The luminescent and magnetic properties of the three pure compounds [Sm(L)(ox)(H2O)]·H2O, [Nd(L)(ox)(H2O)]·H2O and [Nd2(L)2(ox)(NO3)(H2O)3][NO3] were also studied.

We demonstrate theoretically and experimentally that the nonlinear interaction between excitations whose harmonic energies coincide gives rise to a strong coupling that opens a new coherent ultrafast energy relaxation path. Instead of an incoherent decay of excitations, that should take a finite time that depends on the energy difference between the initial state and the final state, the nonlinear interaction allows their coherent superposition and thus an instantaneous transfer of the excitation over energies as large as the electron-volt. Such a situation should be encountered in many systems. We demonstrate that such a model applies also for excitons in single-wall carbon nanotubes (SWCNT) where a strong nonlinear Coulomb interaction occurs between E_11 and E_22 states. This explains a wide panel of observations about optoelectronic properties of the SWCNT and gives a coherent picture of their features like the exciton-line spectral positions, exciton collisions and their ultrafast relaxation, as well as the low light-emission efficiency of the nanotubes.

CeO2 is a promising material for applications in optoelectronics and photovoltaics due to its large band gap and values of the refractive index and lattice parameters, which are suitable for silicon-based devices. In this study, we show that trivalent Sm, Nd and Yb ions can be successfully inserted and optically activated in CeO2 films grown at a relatively low deposition temperature (400 °C), which is compatible with inorganic photovoltaics. CeO2 thin films can therefore be efficiently functionalized with photon-management properties by doping with trivalent rare earth (RE) ions. Structural and optical analyses provide details of the electronic level structure of the films and of their energy transfer mechanisms. In particular, we give evidence of the existence of an absorption band centered at 350 nm from which energy transfer to rare earth ions occurs. The transfer mechanisms can be completely explained only by considering the spontaneous migration of Ce(3+) ions in CeO2 at a short distance from the RE(3+) ions. The strong absorption cross section of the f-d transitions in Ce(3+) ions efficiently intercepts the UV photons of the solar spectrum and therefore strongly increases the potential of these layers as downshifters and downconverters.

Solvo-ionothermal reactions were run between cobalt acetate or zinc acetate and 1,3-bis(carboxymethyl)imidazolium chloride ([(MimCO2H)2][Cl]). These reactions led to two isostructural metal–organic frameworks (MOFs). These two compounds, [(MimCO2)2]2Co and [(MimCO2)2]2Zn, were obtained as single crystals, and their structures were determined by single-crystal X-ray diffraction. These two analogs have a 2D framework consisting of the stacking of layers formed by interconnection of rings containing ligands and metal ions. The structural and thermal properties were analyzed by means of ancillary techniques encompassing FTIR, UV/Vis, NMR spectroscopy, and TGA/TDA measurements. In addition, magnetic and EPR spectroscopic measurements on [(MimCO2)2]2Co showed that the Co2+ ions are in a high-spin configuration with a large magnetic anisotropy relying on a rather unusual distorted tetrahedral coordination. The investigation of the optical properties revealed the photoluminescence of [(MimCO2)2]2Zn.

The luminescence of rare earths in CeO2is sensitized by intrinsic Ce3+ions.

By performing time-resolved optical non-degenerate pump-probe experiments, we study the relaxation dynamics of spin-polarized excitons in wurtzite epitaxial GaN and in nitride nanostructures. Those materials are indeed promising candidates for spintronic applications because of their weak spin-orbit coupling and large exciton binding energy (~ 17 meV and ~ 26meV in bulk GaN, respectively). In epilayers, we show that the high density of dislocations increases dramatically the spin relaxation of electrons and holes through the defect assisted Elliott-Yafet mechanism. That makes the exciton dephasing time very short. In high quality GaN/AlGaN quantum wells, both the exciton-spin lifetime S and the exciton dephasing-time T2 were determined via pump-probe spectroscopy using polarized laser pulses and time-resolved four wave-mixing experiments. The evolution of both quantities with temperature shows that spin relaxation occurs in the motional narrowing regime up to 80 K. Above this threshold, the thermal energy becomes large enough for excitons to escape from the QW. Such measurements demonstrate that GaN-based heterostructures can reach a very high degree of control that was previously mostly restricted to conventional III-V semiconductors and more specifically to the arsenide family.

We demonstrate theoretically and experimentally that four-wave mixing processes obey phase-matching conditions that determine not only the conservation of the photon energy and k-momentum but also the orbital angular momentum of light. We report on time-resolved four-wave mixing experiments performed on a CdTe/CdZnTe quantum well in both noncollinear and collinear configurations with Laguerre-Gauss beams. They demonstrate that the polarization wave which is induced in the material keeps memory of the excitation pulse orbital momentum. We show that in the collinear configuration, the large angular acceptance opens up new horizons for improving the spatial resolution in time-resolved experiments.